1. Field of the Invention
The present invention relates to a secondary battery. More specifically, the present invention relates to a protection circuit board with a PTC device, a secondary battery having the same protection circuit board, and a battery pack comprising the same secondary battery.
2. Description of the Related Art
A secondary battery is a battery capable of repeated use after recharging the battery. The secondary battery has broad spectrum of applications for portable multi-function equipment such as mobile phones, notebook computers, camcorders, and the like.
The secondary battery is composed of a bare cell including a can, an electrode assembly housed inside the can, and a cap assembly assembled with the can. The battery pack is configured by provision of a protection circuit board that is electrically connected to the bare cell. A protection circuit provided on the protection circuit board serves to control charge/discharge of the bare cell.
Between the battery cell and the protection circuit board is provided a protection device such as positive temperature coefficient (PTC) device, thermal fuse, or the like. This safety device is called a secondary protection device, apart from a primary protection device that is provided in the battery cell. The secondary protection device is electrically connected to the bare cell to thereby cut off the flow of electrical current to the outside, when the battery rises to a high temperature or a disturbance of a preset voltage takes place due to excessive charge/discharge. That is, the secondary protection device serves to prevent damage and deterioration of the battery. Such a secondary protection device is usually installed in the bare cell and sensitively responds to temperature changes in the bare cell.
A PTC device, which has been primarily used as the secondary protection device, is now installed in the form of a single component on the protection circuit board. Such a protection circuit board is advantageous in terms of simplified manufacturing processes, but suffers from a variety of shortcomings which will be specifically described as follows.
The protection circuit board is installed with the PTC device in the form of a single component, and therefore the PTC device is not in direct connection with the bare cell. In other words, the PTC device is connected to the bare cell through the protection circuit board, so its heat transfer efficiency is low, which consequently results in performance deterioration of the secondary protection device.
In addition, the protection circuit board should have an electrode lead so as to be electrically connected to the bare cell. Then, a pattern design area is decreased correspondingly to an installation area of the electrode lead that is formed on the protection circuit board. Consequently, the protection circuit board should employ a multi-layered circuit board (at least Layered or more) in order to ensure a predetermined pattern-forming area, thereby increasing the production costs.
Further, the protection circuit board has problem with increasing internal resistance due to the PTC device.